User equipments can communicate with each other using an IP-based voice service provided by an IP Multimedia Subsystem (IMS) network. In order to utilize the IP-based voice service, the user equipments register their locations with the IMS network through an access network such as a mobile network or a wireless data network. The user equipments may frequently change access networks, for example, from the mobile network to the wireless data network, based on network conditions. When the access network is changed from the mobile network to the wireless data network, a seamless handoff of an IP-based voice service from the mobile network to the wireless data network is required so as not to interrupt communication between the user equipments. However, related standards of the IMS network do not guarantee service continuity of the IP-based voice service between the mobile network and the wireless data network.